614 . IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



rounded. An averajare specimen mea.siires : leng-th 22.5 mm., great- 

 est width at midlengtli of the shell 28 mm., length of the hinge- 

 line 32 mm., thickness 17 mm. 



Pedicle valve convex, beak pointed and strongly incurved ; car- 

 dinal area sharply defined, curved ; pedicle jpening triangular, 

 higher than wide. Plications rounded, simple, from nine to thir- 

 teen on the lateral slopes ; mesial sinus distinct, one strong me- 

 dian plication extends its entire length and on either side is one 

 (rarely two) weaker plication which arises by the di\'ision of the 

 first plication of the lateral slope. 



Brachial valve convex, beak pointed, short, and scarcely in- 

 curving over the narrow area; mesial fold well developed and 

 marked by a prominent median furrow which is flanked on each 

 side by a strong plication, other furrows and plications are the 

 counterparts of those on the opposite valve. 



On well preserved specimens fine concentric markings and fine 

 longitudinal lines cover the whole surface, on rubbed or worn 

 shells they may be seen between the plications toward the anter- 

 ior part of the valves. Lines of growth, densely crowded in some 

 cases, also are present. 



Following Hall this species has usually been designated in the 

 Iowa Reports as a variety of S. keokiik or simply as 8. keokuk. 

 It rather closely resembles this species but, as pointed out by 

 Weller, it differs from it "in its smaller size, its more sharply 

 defined and more ang-ular mesial sinus towards the beak, and 

 especially in the narrower and less gibbous umbonal region of 

 the pedicle valve. ' ' Some writers have called it S. littoni confus- 

 ing it with that species due perhaps to the fact that Swallow's 

 S. littoni had never been illustrated before the appearance of 

 Doctor Weller 's MonogTaph. 



Occurrence. — Ste. Genevieve, Pella beds, along Lizard creek 

 and in the Illinois Central railroad cutting, Webster county, 

 Iowa. 



Composita trinnclea (Hall) 



Plate XII, figs. 14-17. 



1858. Terehratula trinuclea Hall, Geol. Iowa, vol. I, pt. II, p. 659, 



pi. 23, figs. 4a-c, 5. 



1914. Composita tnnucJea Weller, Geol. Surv. 111., Monog. I, 



p. 486, pi. LXXXI, figs. 16-45. 



Shell small, four- or five-sided in outline, length and width 

 about equal, greatest ■wddth in front of the middle; an average 



